AU608876B2 - Method of making an economical fiber coupler - Google Patents

Abstract

A method of making an economical fiber coupler comprising providing a glass tube (10) having first and second end portions (18, 20) and a midregion, and a longitudinal aperture (12) extending therethrough. Two suitably prepared glass optical fibers (22, 24), each having a core and cladding, are disposed within the longitudinal aperture, the fibers extending beyond each end thereof. The fibers are held taut to effect a tension therein, and they are glued (30, 32) to each end portion. The midregion of the member is heated, collapsed about the fibers, and drawn to reduce the diameter thereof.

Description

~1.4 1.H 6 ZAXM~~s~d~w1)II~oda~s11111L1.25 11111f .4 -7 -1 177777-T-T---r-r I i o tiq

AUSTRALIA

Patents Act

COMPL

1 ETE SPECIFICATIQI

(ORIGINAL)

6088b d76' M Class Int. Class Application Numiber: Lodged: Complete Specification Lodged: Accepted: Published: Priority Li, -~st RelateO. Art: APPLICANTIS REFERENCE: Berkey 11A Name(s) of Applicant(s): Corning Glass Works 4 4.Address(es) of Applicant(s): 1 44 4 4 4 4 14 Hloughton Park, Corning, New York, UNITED STATES OF AMERICA.

Address for Service is: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 INUSTRALIA Complete specification for the invention entitled: METHOD OF MAKING AN ECONOMICAL FIBEIR COUPLER Our Ref 100884 POF Code: 1602f1602 The following statement is a full description of thin invention, incoluding the best method of performing it k~nown to applicant~(s): 6003g/l-1 1 or othier witness required PHILLIPS ORMONDE AND FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Australia P 17/2/83 BERKEY 11A METHOD OF MAKING AN ECONOMICAL FIBER COUPLER o o o'~O o c~ 00 00 0 o 0 0 J '0 o o 00 0 0 0 0 o 00 This application is a contin -alEion in part of U.S.

Patent Application Serial N r82,678 filed August 7, 1987 arnd is related to Patent Application Serial Number 765,652 enti ed "Method of making Low Loss Fiber Qpt-iC' rO'vIPhr" -h-,ZT D. RKeck et al filed Alut 15, 1 9gR Biz'a::trzund of the ineti:n Certain types of fiber optic systems require couplers in which at least a portior, of the .ic~ht Propagating7 I n a I optical fiber is coupled to one or more output fibers. The O~O present inventiqn relates to such fiber optic couplers and more particular.Ly to a cost eff ctive and reproducible method of making such fiber opt~ic couplers.

It has been known t~hat coupling occurs between two closely spaced cores in a multiple core device. The coupling efficiency increaseg with decrepsing core separation 4 nd, in the case of single-mode cores, with decreasing ccre diameter. A number of couplers that are based on these principles have been developed. S~uch couplers are capable of low loss operation; they typicall1y ~exhibit an excess loss of about 1 dB or less.

Multimode and single-mode couplers have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing the To: The Commissioner of Patents P18/7/78 PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Australia -2claddings together to secure the fibers and reduce the spacings between the cores. Coupling can be enhanced by stretching and by rotating the fibers along the fused length thereof as taught in U.S. Patent Number 4,426,215 to Murphy; however, rotating the fibers is disadvantageeous for certain purposes. Also, a portion of the cladding is sometimes removed by etching or grinding to decrease the intercore distance as taught in U.S. Patent Number 4,449,781 to Lightstone et al. Since the coupling region is fragile and ip exposed to the atmosphere, such couplers must then be provided with a hermetic enclosure. These processes are labor intensive and therefore expensive, they oo may lack long term integrety, and do not always result in couplers exhibiting predetermined desired coupling 15 characteristics. Such disaivantages are particularly o apparent in the manufacture of certain single-mode couplers wherein the coupling core sections are to remain parallel to each other to insure that -he propagation constants are matched and of certain single-mode couplers which must SoC possess optical characteristics such as polarization s o retention.

0 o o Although rmst z:'uplers are made by applying heat oo directly to the fibers to be joined, U.S. Patent Number 0s 3,579,316 to Dyott et al. teaches a method wherein the fibers are first inserted int= a capillary tube where the 0 oo ends may overlap. The capillary tube is formed of a glass 0 having a refractive index lcwer than that of the fiber cladding material. Heat is applied to the capillary tube in the vicinity of the fiber overlap and the tube is stretched until the diameter thereof approximates that of the original fibers. The original cores of the pulled out part become vanishingly small, their stretched diameters being only about 1/100 the original diameters; the cladding of the original fibers becomes the core of the coupling section. Such a long thin ccpler is very cumbersome and fragile. Furthermore, such a coupler is lossy since the original cladding takes the place of the vanished cores.

A

-3 In the region of the coupler where the fiber cores taper from their "vanishingly small" size to their full size, an insufficient amount of power is transferred from the cladding back to the core. Moreover, it is difficult to maintain the cores straight and parallel to one another when fiber s are inserted into a tube which is then stretched unless specific steps are taken to position the fibers. Such non-linear coupler cores can lead to decreased coupling efficiency in single-mode couplers.

Japanese published application 60-140208 teaches a coupler formed by pre-twisting a pair of fibers, inserting them into a quartz tube, and heating and drawing the central part of the tube to reduce its diameter. Resin is o then applied to the ends of the tube to seal the fibers 6 15 thereto. This coupler has the following disadvantages.

,o During the collapse of the tube onto the fibers, the o capillary tube is not evacuated and the fibers are not held taut. The fibers therefore meander in the tube, thereby preventing the achievement of a predetermined coupling when the tube is elongated by a predetermined o length. This also makes it difficult to achieve low _a cupler loss. The fibers are pre-twisted in order tc provide a sufficient length of fiber-to-fiber contact to provide adequate coupling. Such a coupler cannot maintain the polarization of an input optical signal; moreover, it is difficult to make wavelength division multiplexed o couplers with twisted fibers.

Summary of the Invention T i- St hFPrp.fenre., n nhjnt't cf t-hp prLqspnf- ni n vpni.n n to provide a method that overcomes the disadvanta esof the prior art. Another object is to provide a od of making low cost, high optical quality optica'-couplers. A further object is to provide a method making optical couplers that are capable of with ding environmental abuses such as temperature cha es and mechanical influences and yet Seffect a re e and redictable transfer o f_ eegy A9 According to the present invention there is provided a method of making a fiber optic coupler comprising the steps of: providing a glass tube having first and second opposite end portions and a midregion, a longitudinal aperture extending from a first end of said tube to a second end thereof, disposing at least two glass optical fibers, each having a core and cladding, within said longitudinal aperture, said fibers extending contiguous to one another o through the midregion of said tube, Screating a differential pressure across the wall of said tube whereby the pressure within said aperture is lower than that at the outer surface of said tube, heating said midregion of said tube, the combined effect of said pressure differential and said heating causing 9 said midregion to collapse onto said fibers and urge the e fibers into contact within one another, and drawing the central portion of said midregion to reduce the diameter thereof.

If the glass fibers have a coating thereon, a portion o" of the coating intermediate the ends thereof is removed, the uncoated portion of said glass fibers being disposed within I a aa the longitudinal aperture of the tube. The fibers are preferably held taut to effect a tension therein.

The inside of the assembly so formed may be cleaned by 0°o applying a vacuum to one end thereof and flowing through the aperture a suitable fluid such as air or a cleaning solution. The miidregion of the assembly so formed is heated to cause it to collapse around the fibers, and it is drawn down to a predetermined diameter. Collapse of the central portion of the tube is facilitated by creating a lower pressure within the aperture than at the outer surface thereof prior to the step of heating the tube.

The fibers may be held taut during the collapse step by affixing each of them to the first end portion of the tube, pulling that portion of the fibers 39 that extend from the aperture at the second end of the tube to apply a tension to the fibers, and affixing each of the fibers to the second end portion of the tube. The fibers can be affixed to the end portions of the tube by applying glue to the fibers to seal them to the first and second end portions of the tube. The step of applying glue to at least one of the end portions of the tube may comprise applying glue to less than the entire circumfirential region around the fibers, thereby leaving an opening between the aperture and the glue, whereby an access is retained at that end of the :ube to the aperture. The application of glue to the fibers in such a manner that the i aperture is not blocked thereby is facilitated by utilizing I a capillary tube having enlarged, tapered apertures at the ends thereof.

Although low loss couplers have been made by S collapsing the tube onto the fibers and drawing or %o stretching the midregion of tube in a single heating operation, it is advantageous zo separately perform these O steps. If the tube is allowed to cool prior to heating it for the stretching operation, re control can be -exerted over ea h step. A central p;r of. the solid ccllapsed midregion can be stretched, thereby keeping the stretched portions of the optical fibers completely enclosed in the matrix glass of the tube. This improved hermeticity is advantageous since it prevents the stretched portions of the fibers from being adversely affected by water and the like, a factor that can adversely modify the optical i characteristics of the coupler, Access to the aperture for cleaning or evacuating purposes may be obtained by disposing a hollow filament within the end portion of the tube contiguous to the two glass fibers before the glue is applied. After the steps of dollapsing and drawing the central region of the tube, the extending hollow filaments are removed and the resulting apertures or holes sealed.

I i 6 Brief Description of the Drawing Fig. 1 is a cross-sectional view of a glass tube suitable for the purposes of the present invention.

Fig. 2 is a cross-sectional view of the tube of Fig. 1 within which a pair of optical fibers are disposed.

Fig. 2a is a cross-sectional view of one end portion of the tube of Fig. 2.

Fig. 3 is a cross-sectional view illustrating further steps in the method of the present invention.

Fig. 4 is a cross-sectional view illustrating additional steps in the process of the present invention.

00 o Fig. 5 is a cross-sect-nal view illustrating the a 0' collapse of the glass tube around the fibers to form a oo o5 solid midregion.

o.0 Fig. 6 is a cross-sectional view through the solid 0, midregion of Fig. 5 along lines 6-6.

Sor Fig. 7 is a cross-secti r.n ilustration of the fiber coupler of the present invention after it has been drawn .20 down and sealed at its ends.

oo Figs. 8 and 9 are cross-szectnal views illustrating additional rethods of providin.; access to the tube aperture oo during processing.

Fig. 10 is a cross-sectional view illustrating an additional method of providing access to the tube aperture, oo Q and in addition, illustrates a method of evacuating the tube.

Fig. 11 is a cross-sectional view taken along lines 11-11 of Fig. Fig. 12 is a schematic illustration of an apparatus for inserting fibers into the tube.

Fig. 13 is a schematic illustration of an apparatus for collapsing the tube and drawing the midregion thereof.

7 Description of the Preferred Embodiments The drawings are not intended to indicate scale or relative proportions of the elements shown therein.

Referring to Fig. 1, there is provided a hollow glass cylindrical tube 10 having a longitudinal aperture or bore 12 provided along the longitudinal axis thereof. Tube may comprise a capillary tube which may be formed as hereinafter described in detail or as taught in -my U S. pvact V o N. 1 2Z-1 A8C copending applicationA entitled "Capillary Splice and Method", N 0 a82,680 (Borke' filed on August 7, 1987, which application is incorporated herein by reference 0, Tapered apertures 4 and 16 form funnel-like entrances to So longitudinal aperture 12 at end surfaces 18 and respectively. The tapered apertures facilitate the insertion of fibers into aperture 12, since the maximum cross-sectional dimension thereof may be less than 400 m.

The softening point terperature of tube 1C should be lower than that of the fibers that are to be inserted therein. Suitable tube compositions are SiO 2 doped with 1 to 25 wt, B203 and SiO 2 doped with 0.1 to approximately 2.5 wt. fluorine. A preferred composition is Sborosilicate glass comprising SiO 2 doped with 8-10 wt. S2 B203' In addition to lowering the softening point temperature of SiO 2

B

2 0 3 and F also advantageously decrease the refractive index thereof.

S

a Referring to Fig. 2, a pair of optical fibers 22 and 24, each having a core, cladding, and protective coating, extend through longitudinal aperture 12, a sufficient length of each fiber extending beyond each end of tube to make connection thereto, a length of 1 meter having been found to be sufficient. A portion of the coating intermediate the ends of each fiber is removed for a distance slightly shorter than the length of aperture 12.

The fibers are wiped to eliminate residual material. The uncoated portions of the fibers are disposed intermediate end surfaces 18 and 20 of hollow member 10. Preferably, -4 j- c'

"I

8 the uncoated portions of fibers 22 and 24 are longitudinally centered within aperture 12. For convenience and ease of illustration, fibers 22 and 24 are drawn in the figures with a light line within most of aperture 12 and a heavy line from within the ends of aperture 12 to the exterior of tube 10. The light lines represent uncoated portions of the fibers while the heavy lines represent coated portions thereof. Fig. 2a is an enlarged cross-sectional view of one end of Fig. 2 showing coatings 23 and 25 on optical fibers 22 arn, 24 respectively.

The fibers may be maintained parallel to one another within aperture 12 or may be twisted any amcunt including 180° or more as illustrated in Fig. 3. Twisting the fibers is a well known technique that has been used to maintain S the fibers in mutual contact during the step of fusing the fibers together. If vacuum is employed during collapse of 'o tube 10 as herein described, twisting of the 4i:ers is not critical since the vacuum will assist collapsing the tube and maintaining the fibers in longitudinal cntact with each other. It is noted that for certain types of coupling devices, such as WDM couplers and polarizat.:r. rea:ning couplers, the fibers must be kept untwisted and must be maintained parallel to one another.

The assembly comprising tube 10 and the fibers extending therethrough is preferably subjected to a final cleaning step prior to collapsing tube 10 and fusing together the stripped fiber portions. This step is important since small pieces of coating material or other contaminants may remain on the uncoated portions of the fibers after they have been inserted into tube 10. The cleaning step may comprise flowing a cleaning fluid through aperture 12 and over the stripped portions of fibers 22 and 24. The cleaniag fluid may comprise a liquid cleaning solution such as a 30 ammonia solution or a gas such as air. Furthermore, the fibers are preferably held taut during the tube collapse step. A variety of techniques can -LI i LI- 9 be employed to effect the steps of cleaning the aperture and tensioning the fibers; preferred techniques being described in the following specific embodiments.

In a first embodiment, a pair of optical fibers 22 and 24, each having a core, cladding, and protective coating, are suitably prepared by removing a portion of the coating intermediate the ends thereof as described above. The uncoated section is cleaned by wiping with a lintless cloth to remove residual material. Fibers 22 and 24 are fed through longitudinal aperture 12 so that a suitable length extends beyond each end of tube 10 for connection purposes.

The uncoated portions of the fibers are disposed intermediate end surfaces 18 and 20 of hollow member 10 as S shown in Fig. 2, the uncoated portion of fibers 22 and 24 S~ ,5 preferably being centered within aperture 12.

.o o For certain types of couplers, fibers 22 and 24 may then be twisted within longitudinal aperture 12 about 1800 as illustrated in Fig. 3. A hollow glass filarcent Z6 is S inserted into the end portion of tube 10 so that it extends a short distance into longitudinal aperture 12. Fibers 22 S and 24 and filament 26 are then secured to the end portion of member 10 by applying a quantity of glue 3 w.th-.n and about tapered aperture 14. The process is then repeated at oo the other end of member 10 by inserting a second hollow glass filament 28 into longitudinal aperture 12 and applying a quantity of glue 32 to the fibers within and S about aperture 16. While glue 32 is setting or curing, a slight tension is applied to fibers 22 and 24. Glue 30 and 32 may ccisist of any bonding material such as cement, adhesive or the like, UV curable epoxy being preferred.

The assembly so formed is then placed in a suitable mounting device or holder 34, such as a tinners clamp.

Hollow filament 28 may then be connected to a suitable source of vacuum (not shown) illustrated by arrow 36.

Alternatively, a tube connected to a source of vacuum may be placed around the end of capillary tube 10 so that hollow filament 28 and fibers 22 and 24 extend into the pj evacuated tube. If hollow filament 26 is inserted into a liquid cleaning fluid, the fluid is drawn through longitudinal aperture 12 by the vacuum applied to hollow filament 28 whereby it cleans the interior of longitudinal aperture 12 and those portions of fibers 22 and 24 and hollow filaments 26 and 28 that are disposed therein.

If a liquid cleaning fluid is employed, midregion 38 of the assembly so formed is then heated by a suitable heat source 40 as illustrated in Fig. 4, to vaporize the liquid and dry out the assembly. This drying step is not needed if a gas is used as the cleani-.g fluid.

In accordance with one embodiment of the present invention, tube 10 is heated and collapsed onto fibers 22 and 24, and thereafter, the mi.region of tube 31 is heated .1 5 and stretched to bring the fiber cores closer together along a distance sufficient to accomplish a predetermined type of coupling, This is a-somplished by first heating 0 midregion 38 to the softeni- point of the horcsilicate glass tube 10 by means of heat source 40, which may comprise an oxygen-hydrogen burner, a gas-oxyger. burner, or «o the like. Burner 40 may reain stationary or it ray aD atraoerse mldregion 3S in th- s-zaction toward 4a:u'- s,7arce 36 as shown by arrow 41 in Fig. 4. It is an optional S feature of the tube collapse step to apply a vacuum source to both hollow filaments 26 and 28, in which case the direction of burner traverse is immaterial. The step of a subjecting midregion 38 to heat source 40 causes the material of tube 10 at midregion 38 to collapse about fibers 22 and 24 as additionally illustrated in Fig. Fig. 6 illustrates the collapsed midregion 38 of tube about fibers 22 and 24 along line 6-6 of Fig. 5. The portion described as midregion 38 becomes a solid region that is preferably free of air lines, bubbles, or the like.

The assembly so formed is removed from holder 34 and placed in a precision glass working lathe illustrated by members 42 and 44 in Fig. 5. The solid midregion 38 is then subjected to the flame from an oxygen-hydrogen burner R- ll 11 46 until a portion of the solid midregion 38 is heated to the softening point thereof. If the entire midregion were stretched, the end portion of the light coupling region of the fibers could be exposed to the aperture, Stretching only the central portion of the collapsed midregion ensures that the coupling region of the fibers will be embedded in the matrix glass of the capillary tube. The flame is removed and the softened portion of midregion 38 is pulled or drawn down by action of the glass working lathe to reduce the diameter thereof as illustrated by region 48 of Fig. 7. The diameter of drawn down region 48 will vary as a function of various fiber and operational parameters.

The ratio of the drawn down diameter of region 48 to the starting diameter of midregion 38 (the draw down ratio) is l 5 determined by the optical characteristics of the particular o device being made. It is well known that such draw down ratios are a function of the ratio of the signal split between the fibers, the refractive index difference between the tube and the fiber cladding, the outside diameter of the fiber cladding, the diameter of the fiber core, signal operating wavelength, cutoff wavelength, the tolerable excevs 1 and the Xir. r. pcfcrred ra:ge of draw down ratios is between about 1/2 to 1/20; however, couplers can be made having draw down ratios outside this range.

As illustrated in Fig. 5, the portion of member held by glass working lathe member 42 is held stationary while the portion of member 10 held by lathe member 44 is Straversed in the direction of arrow 50 to obtain drawn down region 48. In practice, such a pull down or draw down takes approximately 1/2 second. Alternative drawing techniques involve the movement of lathe member 42 in the same direction as that in which member 44 moves or in a direction opposite that in which member 44 moves, The assembly would not need to be rotated if the draw down portion of midregion 38 were heated by a ring burner which would uniformly heat that region around its periphery. The draw down method would otherwise be the 12 same. In the embodiment wherein a ring burner is employed, the step of collapsing tube 10 onto fibers 22 anc" f and the step of forming drawn down region 48 may be pr ,ied on the same apparatus. If the collapse and stretch perations are performed in the same apparatus, it is preferred that tube 10 be allowed to cool prior to being reheated for the stretch step. This temporal separation of the two steps results in better process control and therefore better reproducibility. Furthermore, tube 10 can be disposed in any orientation including vertical and horizontal during the tube collapse and/or drawing operations.

After the draw down, the exposed ends of hollow filaments 26 and 28 are removed by breaking them off at the 1: surface of glue 30 and 32, and the apertured ends thereof are sealed with quantities 54 and 56 of glue as heretofore described. The resulting assembly comprises fiber optic coupler 52 of Fig. 7. The couplor can be further processed by ackaging, not shown, for additional stiffness if dosired-d Coupler 52 functions to couple a signal in optical fiber 22 to optical fiber 24 and vice versa.

n ac:rdane with the above-described embodiment, the steps of collapsing and stretching are separately performed. This is advantageous since more control can be exerted over each step if the tube is allowed to coo prior to heating it for the stretching operation. A central portion of the solid collapsed midregion can be stretched, Sthereby keeping the stretched portions of the optical fibers completely enclosed in the matrix glass of the tube, This improved hermeticity is advantageous since it prevents the stretched portions of the fibers from being adversely affected by water and the like, a factor that can adversely modify the optical characteristics of the coupler.

Low loss couplers have also been made by an alternative embodiment wherein the steps of collapsing the tube onto the fibers and drawing or stretching the midrogion of the tube are performed in a single heating drawing tne centraL poruILu1 ui. O.±u LiLLLuiLcy.u Lu ijuuue the diameter thereof.

13operation. In accordance with this modified embodiment, the fibers are inserted into the capillary tube and are glued taut to the ends thereof such that there are access openings to the aperture. This assembly is placed in a precision glass working lathe as described above. A flame is applied to a small portion of the midregion until the softening point of the materials is reached, and the heated section is stretched. For a given amount of coupling, the amount of tube elongation is greater in this embodiment 1C than in that embodiment wherein the tube collapse and the medregion stretching steps are separately performed.

Finally, glue is applied to the ends of the device to close the openings to the aperture.

-o The disadvantages of this embodiment are a reduction in hermeticity and an adverse affect on manufacturing reproducibility, i.e. stretching to a predetermined length Sdoes not always result in the desired coupling S characteristics, However, this embodigtent has some advantages over other methods. The method is simpler in that it can be performed without vacuum and the separate tube collapse step is eliminated. Low loss ccuplers have been formed by this mrthod, device losses as lC as 0.05 dB 1300 'm having been measured.

In another alternative embodiment a hollow fiber is employed in only one end of tube 10. Such an embodiment is similar to that resulting in the formation of coupler 52 except that the internal cleaning step described will not 'o be practical.

The hollow fibers are eliminated entirely in the embodiments of Figs. 8 through 10, wherein elements similar to those of Fig. 2 are represented by primed reference numerals.

The initial steps needed to form the embodiment of Fig. 8 are the same as those employed to form that of Fig.

3. A filament similar to filament 28, which may be hollow or solid, is inserted through tapered aperture 16' so that it protrudes a short distance into aperture 12' as shown in 6003q/1 1- 1 1 3. After the uncoated portions of fibers 22' and 24' I. .,een centered in aperture 12', a quantity of glue 58 is applied within and about tapered aperture 16'. As shown in this figure, the glue advantageously extends into aperture 12' a sufficient distanice to contact the glass cladding of fibers 22' and 24'. As glue 58 begins to set or cure, its viscosity becomes sufficiently high so thnat removal of the extra filament will leave an aperture 59 into which bonding material 58 cannot flow and close. An aperture similar to aperture 59 could also be formed in the glue located at the opposite end of tube 10' .To clean and/or evacuate aperture 12', a vacuum attachment tube may be placed around the pheriphery of tube 10' at end surface thereof as described above.

13 In the embodiment of Fig. 9 glue 61 ccmpletely seals tapered aperture 16'. A radial bore 62 near the end 20' of tube 10' provides access to aperture 12' for cleaning/or Sevacuating purposes, Apert'jre 2'can be e%,actate. through 'ore 62 by attaching an annular vacuum fixtt.urQ 64 to tube 10'1 so that bore 62 opens into annular slot 63., Arrow V indicates tbit a vacuum source .is connected to finture 64, A radial bore similar to bore 6Z ca.- be -o-d at the opposite end of tube 10' for purposes such as L.Lpplying cleaning fluid or gas to aperture 12' and/'or evacuating 2 5 aperture 12'.

F,)r a coupler manufacturing process to consistantly produce couplers having predetermined optical characteristics, all of the process, steps, including the step of inserting the fibers into the capillary tube should be uniformly performed on each coupler made. Discussed along with a description of the method of for-ming the embodiment of Figs. 10 and 11 is a preferred fiber insertion meth od which enhances process uniformity. 11C is advantageous to employ a fiber insertion station which meets the following criterion. The mechanism which is to hold the fibers should be properly aligned since the fibers are preferably kept untwisted and straight. Means should L -JiL..IJIJ. CI.UIPy CE )uLd.Lt- Lt-iy1 Lnereor ana rusing the S- j be provided for holding the fibers under a slight tension during the gluing step to eliminate the occurrence of fiber slack or sag during further processing steps, especially during the step of collapsing the capillary tube onto the fibers. The appearance of slack in one or both of the fibers could cause the resultant device to exhibit an excessive loss. The area around the station should be free from excessive dust and other particulates that could be drawn into the capillary tube and lodged inside, since seeds could result from such particulate matter during the collapse and redraw steps. The excessive attenuation that can result from such seeds could render the coupler useless.

A suitable fiber inserticn station, which is shown in Fig. 12, comprises aligned blocks 67, 74, 76, 79, 82 and 83. Rubber surfaced clamps 70 and 71 are capable of retaining optical fibers against block 67. Similar clamps o. 84 and 85 are associated with block 83. The -lamps, which S6 are spring biased against the blocks, can be withdrawn from contact with the blocks by depressing a handle connected thereto. Block 74 contains spaced grooves 72 and 73 that are aligned with grcv-:z S0 and 81 of bli:ok S A single groove 75 in the surface of block 76 is aligned with similar groove 78 of block The illustrated grooves may be U-shaped and may have a width that is just sufficient to slidingly accomodate the fiber or fibers that are situated therein.

Two lengths 22'and 24'of coated optical fiber are severed from a reel of fiber. An end of each of fibers 22'and 24' is secured by clamps 70 and 71, respectively.

The entire lengths of the fibers are wiped with a lintless cloth dampened with a suitable cleaning solution such as ethyl alcohol.

There is selected a capillary tube 10', the aperture of which is preferably just large enough to a.ccept the coated portions of the optical fibers. Such a relationship between the coated fibers and the aperture prevents the original cladding takes the place of the vanished cores.

S- 16 -16 ends of the fibers from twisting within the tube. As illustrated in Fig. 11, certain hole cross-sectional shapes such as diamond, square and the like facilitate the proper alignment of the fibers in the tube. The aperture diameter should not be so small that it is difficult to thread the fibers therethrough, since this could cause the coating to smear on the inside of the tube. The smeared region of the tube could cause the resultant coupler to contain seeds that would degrade the coupler's performance. In order to facilitate easy movement of the tube along the fibers, a small amount of ethyl alcohol may be squirted into the tube. This functions as a temporary lubricant which will readily evaporate. The capillary tube is threaded onto the o fibers and moved to approximately the position shown o 004 0 ,*15 adjacent block 76. The fibers are plled slightly so that they are under some tension and the remaining ends thereof are then restrained by clamps 84 and 85. A mechanical 0 n t stripping tool is utilized to remrrve a portion of the a ,s coating from each fiber at a locaticn thereon between tube 10' and block 79. The length of the stripped section of Sfiber is slightly shorter than the length of the capillary t Zee; apert,'.re tc all:w the a.ing t extend into both ends of thereof, thereby properly positioning the fibers within the aperture cross-section. The lengths of the stripped regions should be about equal, and those regions should be adjacent one another.

Using a dampened lintless cloth, the two fibers are grasped at the left end of tube 10' and are wiped firmly, the cloth being moved away from the tube and across the stripped regions. This step removes any loose material generated by the coating stripping step and leaves a pristine surface on the stripped regions of the fibers.

The fibers are then placed into grooves 75 and 78 which help to hold the fibers straight and adjacent one another.

Clamp 84 is released and then reclamped after fiber 22' has been retensioned; fiber 24' is then similarly retensioned.

17 The capillary tube is moved toward block 79 and positioned such that it is centered over the stripped region as shown in Fig. 10. A small amount 87 of glue is applied to one side of fibers 22' and 24' to attach them to one side of tapered aperture 16' while leaving an opening 88 which penmits access to longitudinal aperture 12' between glue 37 and the remainder of the tapered aperture 16'. A drop 89 of glue is similarly applied between the fibers and tapered aperture 14', leaving aperture access opening 90 between glue 89 and tapered aperture 14' Depending upon the size of the capillary tube aperture, it can be difficult or even impossible to glue the fibers to the tube end portions without blocking the aperture unless the tube is provided with tapered apertures 14' and 16' Openings 88 and 90 permit the flow of fluid through aperture 12' during the final wash, and also permit the evacuation of aperture 12' during the collapse of tube S If the glue is a UV light curable epoxy, UV light source 86 e is directed on the first applied drop of upoxy to cure it before the second drop is applied to the remaining end.

I After the second drop is applied, soirce 86 is moved as Sp indicated by the arrows and directed on:o the second drop.

The pigtails or sections of fiber extending from the ends of tube 10' can be color coded. At this time the fibers within the capillary tube are visually checked for Sinternal twists. A twist of mo:e than 1800 can be seen by the naked eye. Also, a laser beam can be launched into that end of fiber 22' protruding from clamp 84. If there I is no twist present, the light emanates from that end of fiber 22' protruding from clamp 70. An orientation mark can be placed on the upper surface of tube 10' so that the fibers can be oriented in the same manner with respect to the draw apparatus for each coupler that is made, thereby ensuring every coupler preform is subjected to uniform process conditions.

A preferred apparatus for performing the tube collasing and stretching steps is shown in Fig. 13.

,I -i n S hai Sis i et ha 'I 3 0 ~fibr2'poruigfo lap7.A rinainmr pulling that portion of the fibers 39 -4- 399

AB

-18 Chucks 92 and 93, which are used to secure the coupler preform in this apparatus, are mounted on motor controlled stages, which are preferably controlled by a computer. The numerals 92 and 93 are also used to designate the stages.

Symmetry is an important requirement for the collapse and stretch steps; therefore, chucks 92 and 93 must be aligned to prevent the occurrence in the coupler of an offset which can adversely affect device loss and which can also advew'sely affect coupler bidirectionality, that characteristic whereby coupler output performance is substantially uniform regardless of which end of a fiber is selected as the input port. Coupler bidirectionality is also enhanced by locating the burner centrally along the 0o coupler preform so that it heats the preform evenly. A symmetrically designed burner such as r'rig burner 94 is 0, suitable for evenly heating the capillary tube midregion.

Heat shield 95 protects the apparatus icated above the burner.

oo Coupler preform 91 of Fig, 10 is inserted through ring burner 94 with the orientation mark facing a predetermined direction. The preform is clamped tc the draw chucks, and vacuum attitchv.ents 96 and arc atta:xed to the ends thereof. Vacuum attachment 96 which is shown in cross-section in Fig. 10, may comprise a short, somewhat r, °25 rigid section of rubber tube having a vacuum line 97 extending radially therefrom. One end of a length of thin rubber tubing 98 is attached to that end of vacuum o attachment 96 that is opposite preform 91; the remaining end of the tubing extends between clamp jaws 99. Upper vacuum\ attachment 101 is similarly associated with line 102, tubing 103 and clamp jaws 104. Fibers 22' and 24' extend from tubing 98 and 103.

Vacuum is applied to the lower portion of coupler preform 91 for a time sufficient to wash aperture 12' by clamping jaws 99 on tubing 98. The upper line is vented to air during this time by leaving clamp jaws 104 open. This "air wash" pulls from aperture 12' any loose debris which Leau±LtLiy aperTures or noieg sealea.

rl II 19 has accumulated therein during the fiber insertion step.

Jaws 104 are then clamped against tubing 103 to apply vacuum to the upper portion of preform 91.

The capillary tube collapse step entails heating the coupler preform with the flame from ring burner 94 for a short period of time, typically 25 seconds, to increase the temperature of the midregion of the tube to the softening temperature. With the assistance of the differential pressure on the tube, this causes the matrix glass to collapse onto the fibers and urges them into mutual contact. The tube matrix glass surrounds the fibers and fills the aperture to form a solid structure, thereby eliminating airlines in the coupling region. The aperture is preferably evacuated from both ends thereof during the collapse step. The longitudinal length of the region that is to be collapsed is determined by the flame temperature, as determined by the flow of gases to the burner, and the Oo!n" time duration of the flame.

o, The central portion of the collapsed midregion of the tube can be stretched without removing the device from the apparatus in which the tube was collapsed. After the tube ccols, the fl-e is reignited, and cnte of tnh collapsed region is reheated. The flame duration for the stretch process, which depends upon the desired coupler 0 025 characteristics, is usually between 10 and 20 seconds. The shorter heating period for the stretch step results in a stretched region that is shorter than the collapsed region.

S After the collapsed tube is reheated, stages 92 and 93 pull in opposite directions until the coupler length has been increased by a predetermined amount. If properly aligned apparatus is employed and if the process parameters are carefully controlled, all couplers formed by the process will possess similar optical characteristics.

The amount of stretching to which the capillary tube must be subjected to achieve a given type of coupler is initially determined by injecting light energy into one input fiber of a collapsed coupler preform and monitoring 20 operation. To accomplish this purpose, one of the fiber pigtails is aligned with a light source, and both pigtails at the other end of the device are coupled to light detectors. The predetermined ratio of the dynamic output powers can be used as an interrupt to cause stages 92 and 93 to stop pulling the sample. After having determined the proper stretching distance to achieve predetermined coupling characteristics, the apparatus can be programmed to move the stages that proper stretching distance during the fabrication of subsequent couplers that are to have said predetermined characteristics.

It is conventional practice to monitor output signals l, to control process steps in the manufacture of optical 0 °15 devices as evidenced by U.S. patents Nos. 4,392,712 and ao 4,726,643, U.K. Patent Application No. GB 2,183,866 A and 0 International Publication No. WO 84/04822. Furthermore, Q°a 0 computers are often employed to in feedback systems which Son automatically perform such monitor and cz;trcz functions.

A suitably programmed PDP 11-73 micro-computer can be utilized to perform these functions, The timing sequences that have been used in the fabricatic:. :f i particular type g' of coupler can be entered in a separate multiple command file that the computer recalls at run-time. The collapse u'a 25 and stretch steps that are required to make that particular coupler can be executed in succession by the computer on each coupler preform to reproducibly manufacture couplers.

SThe process parameters that can be controlled by the computer to ensure coupler reproducibility are heating times and temperatures, flow rates of gases, and the rate at which the stages pull and stretch the coupler preform.

Reproducibility is also a function of the resolution of stages 92 aid 93.

After the coupler has cooled, the vacuum lines are removed from the coupler and a drop of glue is applied to each end of the capillary tube where it flows at least partially into the longitudinal aperture. This produces a hermetic seal and also increases the pull strength of the UIi;UvcLu pu L±IuI1 Ur cne rivers are aisposea Intermeaiate end surfaces 18 and 20 of hollow member 10. Preferably, iS-- 21 devices. The coupler is then removed from the draw and is ready to be packaged.

Although the foregoing description has been related to couplers made from pairs of optical fibers, it will be evident that the invention is also applicable to couplers made from more than two fibers.

The following specific examples utilize glass capillary tubes formed in the manner described in -my u.s PcAt-ent-)t tQo /so4j c co-pending app atin N 082, 67,' filed August 7, 1987.

Glass particulate material was applied to a cylindrical mandrel, consolidated, drawn, and dried in accordance with the teachings of U.S. Patents Numbers Re. 28,029, 3,884,550, 4,125,388 and 4,286,978, all of which are hereby ea expressly incorporated herein by reference. More 15 specifically, the particulate material was deposited on a o mandrel to form a porous, cylindrically-shaped preform.

The mandrel was removed and the porous preform was consolidated to form a tubular glass body which was heated and redrawn to an outside diameter of about 2.8 t, 3 mm.

One end of the resultant capillary tube was attached to a source of air pressure, and while the tube was rc=ated, a a a flame was directed onto the tube at spaced intervals. The air pressure within the tube caused a bubble to be formed at each region of the tube softened by the flame. The tube was scored at the center of each bubble and then severed at each score line to produce a capillary tube having tapered apertures at each end thereof. My--,oe nd+n--p-gta-rt U1S, p c i4 d o I'6,tsoqcZ6, ti' S iled August 7, 1987 (Berkey 12) teaches a method of producing apertures of non-circular cross-section by shrinking the tube onto a carbon mandrel of desired cross-section and then burning out the mandrel; this application is incorporated herein by reference.

Example 1 Reference will be made to Figs. 1-7 during the description of this example. Capillary tube 10 was formed as described above; it had an outside diameter of about 2.8 mm, a longitudinal aperture diameter of 400 mn, and had a length of 5.1 cm. Tube 10 was formed of a borosilicate *lass containing 8 wt B10 3 Two single-mode optical rurtnermore, tne riDers are preferably held taut during the tube collapse step. A variety of techniques can ll Iill -22fibers having an outside diameter of 125 um were each cut to a length of approximately 2 m. Each fiber comprised a core, a cladding and a urethane acrylate resin coating. A commercially available mechanical stripper was used to remove the resin coating from approximately 3.8 cm (1 1/2 inches) of the central portion of each fiber.

The uncoated portions of the fibers were wiped with a lintless cloth to remove residual matter, and the fibers were pulled through longitudinal aperture 12 until the uncoated portions of the fibers were approximately centered Stherein, A hollow glass fiber 28 having an outside diameter of approximately 125 um was inserted approximately 0.3 to 0.6 cm (1/8 to 1/4 inch) into end 20 of the tube. A 0 0 S° quantity of Norland UV curable glue was disposed within 15 tapered aperture 16 about the three fibers and cured by exposure to UV light for about 1 minute. In this manner the optical fibers ard filament 28 were rigidly affixed to the end of tube ,o The two optical fibers were then twisted 180" within 0 aperture 12, and a second hollow fiber 26 was inserted approximately 0.3 to 0.6 cm into the other end of longitudinal aperture 12. A slight tension was arried tc the twc fibers and a drop of UV curable glue was applied to the fibers within tapered aperture 14. The glue was cured as described above. The assembly formed was mounted in a tinner's clamp that was modified by cutting away the central portion and one end portion of the clamping region such that when the coupler assembly was mounted, midregion 38 and one end surface 18 was exposed. A tube connected to a vacuum source was connected to one end of the capillary tube such that the optical fibers and hollow filament were disposed inside the evacuated tube. In this manner, longitudinal aperture 12 was evacuated through hollow filament 28. Hollow fiber 26 was inserted into a beaker of 30% ammonia solution. The ammonia solution was sucked into aperture 12 whereby the aperture and the outside surfaces of the optical fibers were cleansed for approximately be placed around the end of capillary tube 10 so that hollow filament 28 and fibers 22 and 24 extend into the HI I l l I II ILI I ]17 23 seconds. Hollow fiber 26 was then removed from the beaker of cleansing solution. After as much of the liquid as possible was removed from aperture 12 by the vacuum source, a burner was directed at tube 10 for about 20 seconds to assist in drying out the interior thereof.

The midregion 38 of tube 10 was then heated to the softening point of the borosilicate glass by an oxygen-hydrogen burner whereupon the glass collapsed around the optical fibers within the longitudinal aperture. The flame was then traversed through the midregion in the direction of the vacuum source so that as the material of o the tube collapsed about the optical fibers; residual 1 matter within the longitudinal aperture being sucked out by S 15 the vacuum. In this manner a solid midregion was formed "oeo" free of air lines or bubbles.

S. The assembly so formed was then .removed from the modified tinner's clamp and placed in a precision glass working lathe. The lathe was a Heathway glass work-ng o20 lathe having a computer controlled pull down or drawn down o mechanism. The flame from an oxygen-hydrogen gas burner was then applied to a small portion of the solid midregion Suntil the softening point of the materials was reach-d, whereupon the computer controlled pull down apparatus stretched the heated section for an interval of o o approximately 0.5 second. The diameter of the pulled down section was approximately 0.7mm.

Thereafter, hollow filaments 26 and 28 were broken off, and UV curable glue was applied to the ends of the device to cover the resultant holes. The assembly was then packaged within a stainless steel tube for stiffness.

Signal losses measured on the coupler so formed were typically in the 0.05 to 0.7 dB range at 1300 u.m wavelength. This produced a 50:50 signal split in the 3 3 f,.fibershavinga-,1200.zm,.cutoff wavelength...- Example 2 There was provided a capillary tube 10 of the type described in Example 1. Two single-mode optical fibers of members 42 and 44 in Fig. 5. The solid midregion 38 is then subjected to the flame from an oxygen-hydrogen burner 24 the type described in Example 1 were prepared in accordance with that example. After the uncoated portions of the fibers were wiped, they were pulled through longitudinal aperture 12 and the uncoated portions were approximately centered therein. A quantity of Norland UV curable glue was carefully placed between the fibers and one side of tapered aperture 16, leaving a small opening to aperture 12. The glue was cured by exposure to UV light for about 1 minute. In this manner the optical fibers were rigidly affixed to the end of tube 10. After a slight tension was applied to the two fibers, a drop of UV curable glue was S carefully applied and cured, as described above, to rigidly S adhere the fibers to tapered aperture 14.

The assembly so formed was placed in the precision glass working lathe of Example 1. The flame from an oxygen-hydrogen gas burner was then applied to a small portion of the midregion until the softening point of the materials waz reached, whereupon the computer controlled o" pull down apparatus stretched the heated section for an or,0 interval of approximately 0.6 second. The amount of elongation of the capillary tube was about 4 cm, about twice the amount of tube elongation needed in E pple 1.

The diameter of the drawn down section was approximately 0 .4 mm. UV curable glue was applied to the ends of the device to close the openings to the aperture.

Devices formed by this method functioned as 3 dB couplers, that is, it produced a 50:50 signal split, Signal losses measured on these devices were as low as 0.05 dB 1300 umn.

Example 3 Employing the apparatus of Figs. 12 and 13, the following steps performed in order to fabricate a single-mode 3 dB coupler. Reference will also be made to .theoupler preform igs.,0 and 11.. Two-l engths 22 1and& 24' of coated single-mode optical fiber were severed from a which would uniformly heat that region around its periphery. The draw down method would otherwise be the 25 reel of fiber. The optical fibers had a diameter of 125 im, and the diameter of the coated fiber was 160 Jim. The length of each piece of fiber was about 2 meters. The ends of the fibers were secured by clamps 70 and 71, and the fibers were wiped with a lintless cloth dampened with ethyl alcohol.

Capillary tube 10' had an outside diameter of about 2.8 mm and a length of about 4.12 cm. The longitudinal aperture was diamond-shaped, each side of the diamond having a length of about 310 4m. Tube 10' was formed of a orosilicate glass containing 8 wt B 2 0 3 The minimum cross-sectional dimension of the diamond-shaped aperture was just large enough to accept the coated portions of the optical fibers in the manner illustrated in Fig. 11. A small amount of ethyl alcohol was squirted into the capillary tube which was then threaded onto the fibers and moved to approximately the position shown in Fig, 12, The fibers were pulled slightly and the remaining ends thereof were clamped, A section of coating about 3.2 cm (1.25 inch) long was removed from each fiber at a location thereon between tube to and block 79, The length of the stripped sectionr of fiber was s~-l:.hy shorter than the length of the capillary tube aperture, The two fibers were again wiped with a lintless cloth that had been dampened with ethyl alcohol to remove loose material generated by the coating stripping -tep. The fibers were placed into grooves 75 and 78; they were then retensioned and restrained by clamps 84 and Tube 10' was centered over the stripped region as shown in Fig. 10, and the fibers were tacked to the ends of the tube as described above using Dymax 911 UV curable adhesive. A small amount 87 of the adhesive was carefully applied to one side of fibers 22' and 24' at each end of the tube to ensure the presence ,of openings g8and-90T-he. adhesive was exposed to a Dymax PC-3 UV light source for thirty seconds at each end of the tube. The fiber pigtails extending from the coupler preform were color coded. At tube onto the fibers and drawing or stretching the midregion of the tube are performed in a single heating 26 this time the fibers within the capillary tube were visually checked for twists. Also, a beam of HeNe laser light was launched into that end of fiber 22i protrtvding from clamp 84. The radiation of light from the remaining end of that fiber indicated that no partial twist was present. An orientation mark was placed on the upper surface of tube Coupler preform 91 was inserted through ring burner 94. With the orientation mark facing the operator, the ends of the preform were secured in chucks 92 and 93.

I oVacuum attachments 96 and 101 were attached to the preform ends as shown in Fig, 13. Jaws 99 were clamped on tubing 98 to apply a vacuum to the lower portion of coupler o preform 91 while the upper end of the preform was vented.

This "air wash" was continued for approximately thirty o seconds, Jaws 104 were then clamped against tubing 103 to apply to the upper portion of preform 91 a vacuum that was allowed to stabilize at approximately 53 cm (21 inches) of Hg.

The ring burner was turned on for about 25 seconds to increase the temperature of the midregion of the tube to the sof ening temperature cf the borosilicate glass, This caused tube to collapse onto the fibers along a section of the tube about 0.6 cm long. After the coupler preform cooled for about 30 seconds, the flame was reignited, and the collapsed region was reheated for about 16 seconds.

Stages 92 and 93 moved in opposite directions to increase the coupler length by about 1.1 cm. All of the process steps performed in the tube collapse step and the stretch step were performed under the control of a PDP 11-73 micro-computer.

After the coupler had cooled, the vacuim lines were removed from the coupler, and a drop of Dymax 304 adhesive was applied to. each end =of tha~capi~l1arytube, and -was exposed to UV light for 30 seconds. The coupler was then removed from the draw.

it protrudes a short distance into aperture 12' as shiown in I 27 -I This process typically produced 3 dB coxuplers that operated at a predeteirniined wav.elength such as 1300 run.

Median excess device loss was about 0.3 dB, and the lowest measured loss was 0.01 dB.

nO a" o OtJ 00 00 o 00t 0 00 0 0 o o,*ip~08h 40.040 7A0~ 00. rO. ~0 M0000t0004t0~!&0.4 0

Claims (24)

1. A method of making a fiber optic coupler comprising the steps of: providing a glass tube having first and second opposite end portions and a midregion, a longitudinal aperture extending from a first end of said tube to a second end thereof, disposing at least two glass optical fibers, each having a core and cladding, within said longitudinal aperture, said fibers extending contiguous to one another s through the midregion of said tube, creating a differential pressure across the wall of said tube whereby the pressure within said aperture is lower 0 4 .n than that at the outer surface of said tube, heating said midregion of said tube, the combined effect of said pressure differential ond said heating causing said midregion to collapse onto said fibers and urge the fibers into contact within one another, and drawing the central portion of said midregion to reduce the diameter thereof.

2. The method of claim 1, wherein the steps of heating and drawing comprise heating said tube midregion to cause it to collapse around said fibers and form a solid midregion, and thereafter heating at least a portion of said solid midregion and drawing the central portion of said midregion.

3. The method of claim 1 or claim 2, wherein the step of disposing comprises affixing each of said fibers to said first end portion of said tube, pulling that portion of said fibers that extend from said aperture at said second end of said tube to apply a tension to said fibers, and affixing each of said fibers to said second end portion of said tube.

4. The method of claim 3, wherein the step of affixing said fibers to the end portions of said tube comprises applying glue to said fibers to seal said fibers to said first end portion of said member and applying glue to said fibers to seal them to said second end portion of said member, the step of applying glue to at least one of the end pojtions of said tube comprising applying glue to less than 391 -28- AB 1 7yjt 'i il i- i 1Y--i_ the entire circumferential region around said fibers, thereby leaving an opening between said aperture and said glue, whereby there is retained an access to said aperture at that end of said tube.

The method of any one of claims 1 to 4, wherein the step of creating comprises evacuating said aperture prior to the step of heating said tube to collapse the central portion thereof.

6. The method of any one of claims 1 to 5, wherein said at least two glass fibers have a coating thereon, said method further comprising the step of removing a portion of said coating from a portion of each of said fibers and disposing the uncoated portion of each of said glass fibers within said longitudinal aperture of said tube.

7. The method of any one of claims 1 to 6, wherein the range of ratios of the diameter of the drawn portion of said midregion to the starting diameter of said tube is between o approximately 1/2 and 1/20.

8. The method of any one of claims 4 to 7 further comprising a step of applying additional glue to said first and second ends of said tube, to hermetically seal any uncollapsed portions of said aperture.

9. The method of any one of claims 4 to 9 wherein the step of providing a glass tube comprises providing a tube 0 4 having enlarged tapered apertures in said end portions for providing access to said longitudinal aperture from the ends o of said tube, said tapered apertures facilitating gluing of said fibers to said end portions without eliminating access to said longitudinal aperture.

10. The method of any one of claims 1 to 5, wherein said at least two glass fibers have a coating thereon, said method further comprising the step of removing a portion of said coating from a portion of each of said fibers, disposing the uncoated portion of each of said glass fibers within said longitudinal aperture of said tube, and, after the step of drawing, applying to the ends of said tube a sufficient amount of glue that some glue enters said aperture and contacts the stripped portions of said fibers. 39 -29- 13 colla 1 aing and stretching steps is shown in Fig. 13. A 4

11. The method of claim 1 or claim 2, wherein the step of disposing comprises applying glue to said fibers to seal them to said first end portion of said tube, applying tension to said optical fibers within said longitudinal aperture, and applying glue to said fibers to seal them to said second end portion of said tube.

12. The method of claim 11, wherein, prior to the step of sealing said fibers to said first end portion of said tube a hollow glass filament is disposed contiguous to said two glass fibers such that it extends into said longitudinal «o aperture at said first end portion of said tube, the step of applying glue comprising applying glue to said fibers and S; said filament to seal said fibers and filament to said first 0 O end portion of said tube, said filament extending through said glue to provide access to said aperture.

13. The method of claim 11 or claim 12, wherein said at *O o least two glass fibers have a coating thereon, said method further comprising the step of removing a portion of said coating from a portion of each of said fibers and disposing the uncoated portion of each of said glass fibers within said longitudinal aperture of said tube.

14. The method of any one of claims 11 to 13, wherein the step of heating comprises heating the central portion of said tube to at least the softening point of the material thereof, o i to collapse the central portion of said tube around said fibers, thereby forming a solid midregion, and thereafter heating at least a portion of said solid midregion to at least the softening points of the materials of said glass fibers and said tube and drawing at least a portion of said midregion.

The method of any one of claims 11 to 14, wherein the step of creating comprises evacuating said aperture prior to the step of heating said tube to collapse the central portion thereof.

16. The method of any one of claims 12 to 15, wherein, prior to the step of sealing said fibers to said second end portion of said tube a second hollow glass filament is disposed contiguous to said two glass fibers such that it 39 AB "air wash" pulls from aperture 12' any loose debris which extends into said longitudinal aperture at said second end portion of said tube, the step of applying glue comprising applying glue to said fibers and said filament to said said fibers and filament to said second end portion of said tube, said filament extending through said glue to provide access to said aperture.

17. The method of claim 1, wherein the step of drawing comprises allowing said tube to cool, reheating the central portion of said midregion, and stretching said central portion.

18. The method of claim 17, wherein the reheating step is of short;. 4 -ime duration than the heating step.

19. The method of claim 17 or claim 18, wherein the maximum temperature to which said tub- is heated during the reheating step is less than the temperature to which said tube is heated during the heating step. 2*

20. The method of claim 1, wherein the steps of heating and drawing comprise heating said midregion of said tube to a temperature sufficiently high that the relatively low pressure within said aperture causes said midregion to collapse onto said fibers, cooling said midregion, reheating the central portion of said midregion, and drawing the central portion of said midregion to reduce the diameter thereof.

21. The method of claim 20, wherein the reheating step is of shorter time duration than the heating step.

22. The method of claim 20 or claim 21, wherein the maximum temperature to which said tube is heated during the reheating step is less than the temperature to which said tube is heated during the heating step.

23. The method of any one of claims 20 to 22, wherein the step of drawing comprises pulling at; least one of said ends of said tube in a direction away from the other end thereof.

24. The method according to claim 1, substatnially as herein described with reference to any one of the embodiments shown in the accompanying drawings. 39 -31- AB r II .s .L L.A MJL 41 'v A. Ld JLJ L.L I' L L I 1.L L 11j 1.A J JL I k input f iber of a collapsed coupler pref orm and monitoring The method according to claim 1, substantially as herein described with reference to any one of Examples 1 to 3. DATED: 2 JANUARY, 1991 PHILLIPS ORMONDE FITZPATRICK Attorneys For: CORNING GLASS WORKS 0 9 900 9 o 00 0 0 o 0 00' 0 00 00 0 0 0 0~1~0 0 0 a 9 o ~00 9~ 0 o 0 0 0* 0 990 90 0 9 0 1 0 o~ 9 099 ~00 0 0 09 0 000 3071Z A.. -32-